Evoked potential (EP) monitoring assists a surgeon in locating nerves within an obscured surgical field, as well as preserving and assessing nerve function in real-time during surgery. To this end, evoked potential monitoring is commonly employed to capture responses resulting from stimulation of the tissue of interest (e.g., direct nerve, muscle, etc.). Evaluating the aforementioned EP responses allows for immediate assessment of the integrity of the electrical signal path through the tissue of interest. Electrical stimulation can cause excitement of the tissue. During electrical stimulation, a surgical probe applies a stimulus signal near the area where the subject tissue may be located. If the stimulation probe contacts or is reasonably near the tissue, the applied stimulus signal is transmitted to the tissue evoking a response. Excitation of the tissue generates an electrical impulse that is sensed by the recording electrodes (or other sensing device). The recording electrode(s) signal the sensed electrical impulse information to the surgeon for interpretation in the context of determining (EP) activity. For example, the EP activity can be displayed on a monitor and/or presented audibly.
Evoked potential monitoring is useful for a multitude of different surgical procedures or evaluations that involve or relate to nerve conduction. Evaluation of these nerves can assist in preservation of the intended electrophysical function during procedures where there exists a high probability of damage to these tissues. For example, various head and neck surgical procedures (e.g., parotidectomy and thyroidectomy) require locating and identifying cranial and peripheral motor nerves. In some instances, an electrosurgical unit is used during these surgical procedures. Current electrosurgical units include a conductive tip or needle that serves as one electrode in an electrical circuit which is completed via a grounding electrode coupled to the patient. Incision of tissue is accomplished by applying a source of electrical energy (most commonly, a radio-frequency generator) to the tip. Upon application of the tip to the tissue, a voltage gradient is created, thereby inducing current flow and related heat generation at the point of contact. With sufficiently high levels of electrical energy, the delivered energy is sufficient to cut the tissue and, advantageously, to simultaneously cauterize severed blood vessels.
Due to the levels of electrical energy generated by electrosurgical units, systems for evoked potential monitoring experience a large amount of electrical interference when used during electrosurgical procedures. The electrical interference can create a condition where signal levels are obscured. For example, during EP monitoring, electrosurgical activity can create artifacts (e.g., false positives) as well as introduce a significant amount of noise in the evoked potential monitoring system. As a result, current techniques involve using a probe to mute all channels of the evoked potential monitoring system during an electrosurgical procedure. Thus, monitoring of EP activity is typically suspended during operation of the electrosurgical unit. In order for a surgeon to prevent cutting a nerve with the electrosurgical unit, the surgeon will cut for a brief period and then stop cutting such that evoked potential monitoring can be restored. If no EP activity is detected, the surgeon can then cut for another brief period, while pausing intermittently to restore evoked potential monitoring so as to prevent from cutting a nerve. This process is repeated until the surgeon has completed the electrosurgical procedure. Without being able to monitor EP activity, there exists a higher probability of resulting impaired electrophysical function.